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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Fri, 30 Jul 2021 10:33:59 +0800
Message-ID: <CAMZfGtUMBZeo-P48MECO=xM9-apeLMAZZGE3VNsg_u5P523BEg@mail.gmail.com>
Subject: Re: [External] Re: [PATCH 5/5] mm, memcg: always call
 __mod_node_page_state() with preempt disabled
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Matthew Wilcox <willy@infradead.org>,
        Alex Shi <alexs@kernel.org>,
        Wei Yang <richard.weiyang@gmail.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>
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On Fri, Jul 30, 2021 at 9:52 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> On 2021/7/29 22:39, Shakeel Butt wrote:
> > On Thu, Jul 29, 2021 at 5:58 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
> >>
> >> We should always ensure __mod_node_page_state() is called with preempt
> >> disabled or percpu ops may manipulate the wrong cpu when preempt happened.
> >>
> >> Fixes: b4e0b68fbd9d ("mm: memcontrol: use obj_cgroup APIs to charge kmem pages")
> >> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> >> ---
> >>  mm/memcontrol.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 70a32174e7c4..616d1a72ece3 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -697,8 +697,8 @@ void __mod_lruvec_page_state(struct page *page, enum node_stat_item idx,
> >>         memcg = page_memcg(head);
> >>         /* Untracked pages have no memcg, no lruvec. Update only the node */
> >>         if (!memcg) {
> >> -               rcu_read_unlock();
> >>                 __mod_node_page_state(pgdat, idx, val);
> >> +               rcu_read_unlock();
> >
> > This rcu is for page_memcg. The preemption and interrupts are disabled
> > across __mod_lruvec_page_state().
> >
>
> I thought it's used to protect __mod_node_page_state(). Looks somewhat confusing for me.
> Many thanks for pointing this out!

Hi Miaohe,

git show b4e0b68fbd9d can help you find out why we add
the rcu read lock around it.

Thanks.

>
> >>                 return;
> >>         }
> >>
> >> --
> >> 2.23.0
> >>
> > .
> >
>